This invention relates generally to an HDTV receiver and particularly to an HDTV receiver that is also capable of receiving NTSC signals.
In the recently announced Digital Spectrum Compatible High Definition Television System (DSC/HDTV) jointly developed by Zenith Electronics Corporation and AT&T Corporation, spectrum compatibility with co-channel NTSC signals is achieved. Because of the digital nature of the HDTV signal, it is of inherently lower power and the ratio of the HDTV signal power to the NTSC signal power is very low, on the order of 14 dB. In the case of a receiver situated between an HDTV service area and an NTSC service area, the desirable to undesirable signal power ratio for the NTSC signal is about 28 dB and for the HDTV signal is about 0 dB. It is planned that all HDTV receivers will be capable of receiving both HDTV signals and NTSC signals. The present invention is directed to a dual HDTV/NTSC television receiver that automatically selects the appropriate signal when tuned to a "television channel."
In the presently proposed all digital DSC/HDTV system, the video source signal has 787.5 lines per frame, 59.94 frames per second and is progressively scanned at 47,203 Hz (three times the NTSC rate) and has a 34 MHz bandwidth. The aspect ratio is 16:9 and there are 1280.times.720 active pixels in a frame, with the pixels being square. 1575 lines are displayed in 1/29.97 seconds. The HDTV signal has a NTSC like transmission format with one HDTV data frame corresponding in time to one NTSC field. There are also HDTV data fields and HDTV data segments that correspond to NTSC fields and NTSC horizontal lines, although there is no correlation between the HDTV data fields and data segments and NTSC fields and NTSC horizontal lines, respectively, on an individual basis. The HDTV signal has a suppressed carrier and is transmitted with a small pilot to aid in channel acquisition. The spectrum of the HDTV transmitted signal is shown in FIG. 1 with the corresponding carriers of the HDTV and a co-channel NTSC signal illustrated therein. The response curve has a flat portion 32 situated between a Nyquist slope 34, 36 at each end, with the HDTV carrier frequency being positioned 895 KHz from the NTSC picture carrier. This frequency distance is Fcs/4, where Fcs equals 3.58 MHz. The transmitter for the HDTV suppressed carrier arrangement is fully disclosed in U.S. Pat. No. 5,087,975, mentioned above.